1. Field of the Invention
The present invention relates to analog-to-digital converters. More specifically, the present invention is directed to an analog-to-digital (A/D) conversion method employing linear interpolation and an A/D converter using the A/D conversion method.
2. Description of the Prior Art
A/D converters have been widely used in the fields of measurement and control. Recent practical application of A/D converters particularly to digital audio apparatus requires A/D converters having particularly high accuracy and resolution. The successive comparison A/D converter is one of such high accuracy A/D converters. The successive comparison A/D converter samples and holds input signals and provides digital output signals sequentially to a digital-to-analog converter (hereinafter referred to as "D/A converter"). An analog output signal provided by the D/A converter is compared with the sampled and held input signals by a comparator, and a digital signal corresponding to the analog output signal provided by the D/A converter is provided as a converter digital output signal upon the inversion of the output of the comparator. This A/D convertor is complicated in constitution and very expensive due to the employment of the D/A converter. An A/D converter using the charging and discharging of a condenser for A/D conversion, as a so-called single slope A/D converter, is, on the other hand, one of the inexpensive conventional A/D converters. Although such a single slope A/D converter has high resolution, it is not satisfactory in linearity between an analog input and a digital output, i.e., an actual input-output curve showing the relation between an analog input and a corresponding digital output deviates from a straight line showing an ideal input-output relation due to an inaccurate, i.e., non linear, A/D conversion. The nonlinearity of the output characteristics of that A/D converter is attributable, for example, to offset error and/or gain error. It is possible to reduce offset error and gain error to zero in order to minimize the nonlinearity. However, even if the offset error and the gain error are reduced to zero, the input-output characteristics of the A/D converter will still not be the ideal straight line. Thus, it is difficult to improve the linearity of that A/D converter to a satisfactory extent, and the improvement of the circuit constitution of that A/D converter to correct the linearity to a satisfactory extent entails an increase in the cost of that A/D converter and a complication of the circuit.